Anaerobic mesophilic fermentation of sulphate containing yeast
industry wastewaters at laboratory scale with anaerobic sequence batch
reactors (ASBR) was studied. Three different treatment schemes were
investigated--ASBR with and without a polymeric filler and coupled
micro-aerophilic/ anaerobic SBR (CSBR). The optimal concentration of
sludge (total solids 17.3 g [L.sup.-1]) in the reactor and the optimal
reaction time (22 h) were determined. It was shown that in the case of
ASBR efficient treatment characterized by chemical oxygen demand (COD)
removal of 75-82% took place at volume loading rates up to 7.7-8.0 kg
COD [m.sup.-3] [d.sup.-1] and at COD/[(S[O.sub.4]).sup.2-] ratio 8.0. In
optimal conditions the methane content of the biogas was 60%. The best
results for sulphate removal (99%) were achieved in the CSBR with the
concentration of sulphide in the reactor effluent being about 10 mg
[L.sup.-1]. Decreasing treatment efficiency after a long-time
exploitation of these reactors occurred as a result of the formation of
insoluble sediment (presumably CaC[O.sub.3] and
[Ca.sub.3][(P[O.sub.4]).sub.2]).

Wastewater from the yeast industry contains extremely high
concentrations of COD (up to 30 g [L.sup.-1]) and sulphate (up to 4.5 g
[L.sup.-1]). For the treatment of high strength wastewaters anaerobic
digestion appears to be economically more attractive than aerobic
processes. Two important goals are achieved simultaneously in anaerobic
processes: removal of organic matter and sulphates.

Advantages of anaerobic digestion include also relatively low
sludge production and low energy need compared with aerobic treatment.
However, a high sulphate content can lead to the destabilization of the
anaerobic treatment processes due to the hydrogen sulphide formation
[1], especially if COD/[(S[O.sub.4[),sup.2-] is below 10 [2]. Despite
these difficulties anaerobic digestion has been successfully applied for
the treatment of a variety of sulphate-rich wastewaters both at
laboratory and full-scale levels [1]. In comparison with continuous
anaerobic methods, anaerobic digestion is a more flexible and
cost-effective treatment technology [3]. However, there are no reports
in the literature on the treatment of sulphate-rich wastewaters using
anaerobic sequence batch reactors (ASBR).

The main aim of this research work was to study the treatment
process of sulphate-rich high strength wastewaters from a yeast
production plant using ASBR technologies.

MATERIALS AND METHODS

Experimental set-ups

Three different schemes of laboratory-scale experimental set-ups of
ASBR were used. In the first experimental set-up (Fig. 1) a stand-alone
ASBR was used. The ASBR with an active liquid volume of 0.7 L was made
of glass tubing of 0.145 m x 0.075 m (diameter). Plastic tubes were
attached to the filling and drawing ports. Peristaltic inflow pumps
(Zalimp, Poland) were used at rates of 0.48-0.51 L [h.sup.-1] to fill
the reactor, draw off the effluent, and to mix the suspension during the
treatment process. The temperature was maintained constant (35 [+ or -]
2[degrees]C) during the operation by a thermostat. Methane gas
production was measured using a wet gas meter after absorption of
C[O.sub.2] and [H.sub.2]S in a scrubber with 10% NaOH solution.

[FIGURE 1 OMITTED]

In the second scheme the ASBR was loaded with a polymeric filler
(Water Group, Germany): 0.8 cm x 1.0 cm diameter, with a conditional
surface area of 640 [m.sup.2] [m.sup.-3]. The volume of carriers was 0.5
L. Otherwise the experimental set-up was as in the first case.

In the third set-up, a coupled sequence batch reactor (CSBR) where
the anaerobic effluent from the ASBR was recycled through a
microaerophilic system was applied. Mixing in the microaerophilic
reactor was carried out using a magnetic stirrer with regulated stirring
speed (Beco, MM-5, 220 W). The biogas from the anaerobic reactor was
passed to the microaerophilic reactor with the recycling effluent. The
anoxic reactor was open and the temperature of the water was the same as
the temperature of the air in the room (20 [+ or -] 2[degrees]C). The
oxygen concentration was kept at 0.1-0.15 mg [L.sup.-1].

Operating cycle parameters

The operating cycles of the ASBRs in all three set-ups consisted of
three stages: (1) filling and decanting stage--this was accomplished by
replacing the upper layer of the liquid in the reactors (effluent) with
the lower layer adding influent to the bottom of the reactor, (2)
reaction stage with uninterrupted agitation (by suspension recycling),
and (3) sludge settling stage. The total cycle length was 24 h made up
of 23 h of reaction-agitation, 0.5 h at rest for settling, and 0.5 h for
filling and drawing (Fig. 2).

[FIGURE 2 OMITTED]

Sludge used

Two types of seed sludge were used for comparing the efficiencies
of the processes. Anaerobic sludge from the anaerobic digester of the
municipal wastewater treatment plant (WWTP), Tallinn, Estonia, which was
not adapted for the treatment of sulphates, was used in the first two
experimental set-ups, and sulphate adapted anaerobic sludge from
full-scale anaerobic digesters of a yeast plant (AS Salutaguse
Parmitehas, Estonia) was used in the case of the CSBR.

Morphology of the sludge

The morphology of the seed sludge and of the sludge at the end of
the experiments was investigated using microscopy. Sludge samples of 10
mL were washed with 10 mL of distilled water and allowed to settle while
the turbid layer was drained. The procedure was repeated until the water
became transparent. The washed sludge samples were placed into a 3.5 cm
Petry dish and studied.

Microscopy of the structure of the seed anaerobic sludge from
Tallinn WWTP showed that the sludge was of granulated type. The
approximate size of granules was 1.7-2.0 mm. The sludge was mixed with
sand, which seemed to be a good carrier of the sludge granules.

Investigation of the structure of the adapted to the sulphates seed
anaerobic sludge from the Salutaguse yeast plant showed that the sludge
was of flocculated type with a small percentage of single granules. The
approximate size of granules was 0.5 mm. The activated sludge used in
the CSBR experiment for seeding the microaerophilic reactor was
completely flocculated.

Sulphate-rich high strength yeast production wastewaters

The reactors were fed with wastewater from the full-scale yeast
production plant of Salutaguse (Estonia). The chemical composition of
the wastewater was as follows: total COD 14.4-25.7 g [L.sup.-1],
S[O.sub.4].sup.2-] 3.5-5.3 g [L.sup.-1], COD/S[O.sub.4].sup.2-] 2.71-
7.63, total solids 12.9-21.6 g [L.sup.-1], total N 250-350 mg
[L.sup.-1], total P 17.3-48.2 mg [L.sup.-1], trimethylglycine 3.7-4.0 g
[L.sup.-1]. Prior to treatment the wastewater was stored at 4[degrees]C
to prevent premature denaturation.

Sampling and monitoring

The production of biogas in anaerobic reactors, the influent and
effluent pH, and the temperature of the sludge were measured daily. For
the pH determinations a pH meter (E6121, Evicon) was used. Dissolved
oxygen concentration in the microaerophilic reservoir was controlled
twice a day by a conductivity and dissolved-oxygen meter (WTW.GMBH,
M325/Oxi-L5). The COD, total solids (TS), sulphate, and total sulphides
concentrations in the effluent were measured weekly, dissolved
phosphorous and total nitrogen contents were analysed twice a month. In
all cases standard procedures described in standard methods [4] for
wastewater examination were used. Effluent samples were drawn from the
ASBR upon completion of the 30 min decant cycle. Sulphide and sulphate
contents were determined immediately. The COD and phosphorous samples
were frozen before analysis. Completely mixed samples were taken from
the ASBR reactor before and after the end of the experiments and used
for TS determination. The biogas composition was determined with gas
chromatography.

RESULTS AND DISCUSSIONS

The optimal concentration of sludge for the start-up in usual
anaerobic processes is 30-40% of the volume of the reactor, about 15 g
[L.sup.-1] of the sludge [5].

The start-up experiments with three different amounts of seed
sludge were carried out during 47 days. Three identical reactors (first
scheme) were seeded with 30% (TS 12.9 g [L.sup.-1]) of anaerobic sludge
obtained from the anaerobic digester (municipal WWTP, Tallinn, Estonia),
40% (TS 17.3 g [L.sup.-1]), and 50% (TS 21.6 g [L.sup.-1]),
respectively. To allow biomass to adapt to sulphate-rich wastewater the
sludge load was increased step-by-step (5% weekly). During the startup
period the organic load rate (OLR) was gradually increased from 1.4
kgCOD [m.sup.-3] [d.sup.-1] to 7.1 kgCOD [m.sup.-3] [d.sup.-1] (Fig. 3)
and hydraulic retention time (HRT) was changed from 10 and to 2.5 days,
respectively.

[FIGURE 3 OMITTED]

During a month following seeding the OLR of 7.7 kgCOD [m.sup.-3]
[d.sup.-1] was maintained and then the reactors were operated at a
constant OLR value. The efficiency of the treatment process during the
start-up was monitored by biogas production (Fig. 4). A faster start-up
of the reactor inoculated with 40% of sludge than of those inoculated
with 30% and 50% of sludge was observed. The average gas production
rates were 0.96, 0.6, and 0.4 L [d.sup.-1], respectively. Maximum gas
production of 1.6 L [d.sup.-1] was detected for the reactor with 40%
sludge on the 27th day of experiments. On the basis of these results the
reactor with the sludge concentration of 17.3 g [L.sup.-1] was selected
for the subsequent experiments.

As seen from Table 1, during the start-up period the COD removal
efficiency in the reactor was rather low--10-33%, but it increased
toward the end of the start-up period. The change of the operational
parameters during the start-up could be explained by the process of
adaptation of bacteria to a gradual increase of OLR (from 2.16 to 7.7
kgCOD [m.sup.-3] [d.sup.-1]).

[FIGURE 4 OMITTED]

Study of the ASBR process in the treatment of high strength
sulphate containing yeast production wastewaters

The selected reactor with 40% sludge was operated during the
start-up stage until day 39 and then a constant OLR value (7.7 kgCOD
[m.sup.-3] [d.sup.-1]) was applied from 39th to 89th day of the
experiment (see Fig. 3). On the 89th-105th days of operation the amount
of the feedstock was increased to 0.35 L in order to check the maximum
possible loading rate. The maximum OLR applied during this phase was
9.16 kgCOD [m.sup.-3] [d.sup.-1] on the 98th day of operation. At this
OLR inhibition of the treatment process was observed. Gas production
decreased from 3.5 to 0.4 L [d.sup.-1] and the pH of the effluent fell
to 6.01. The experiment was stopped after the process was destabilized
(see data in Table 1).

Stabilization of the pH in the ASBR during the operations is shown
in Fig. 5. During the first month of the ASBR experiment the pH of the
influent was adjusted using 10% NaOH solution. Afterwards the reactor
was operated without any adjustment and the average pH value of the
reactor effluent was 7.4, which indicated a high efficiency of the
anaerobic digestion process. Alkalinity did not vary much during the
study, the average values always remained above 118 mEq [L.sup.-1].
Alkalinity was presumably produced as a result of the reduction of
sulphates to [H.sub.2]S in the presence of organic carbon sources, which
supplied the necessary energy in accordance with the following equation
[6]:

During the operation at a constant OLR of 7.7 kgCOD [m.sup.-3]
[d.sup.-1] a significant increase in the removal efficiency to over 80%
was observed. The maximum treatment efficiency (removal of 84% of COD)
and the maximum biogas production of 3.79 L [d.sup.-1] was reached at
OLR values between 7.7 and 8.0 kgCOD [m.sup.-3] [d.sup.-1]. At higher
OLR (over 8.01 kgCOD [m.sup.-3] [d.sup.-1], days 89-105) the treatment
efficiency decreased.

The data obtained on the treatment efficiency in COD removal were
in agreement with the data published for yeast wastewater treatment
process in [7].

The average sulphate removal efficiency was 95% in the experiment.
Sulphate conversion to sulphide was greater than 80% during the start-up
period. Then during days 39-89 when the OLR was constant no inhibition
was detected and a nearly 100% removal efficiency was observed.
Furthermore, the concentration of sulphates in the effluent did not
exceed 40 mg [L.sup.-1]. Due to the high OLR (9.2 kgCOD [m.sup.-3]
[d.sup.-1], day 98) the conversion efficiency decreased after day 100 to
90%. The data indicated that sulphate reduction was limited at higher
OLR, and higher sulphate concentrations were observed in the influent.
In fact, it has been supposed that for a successful anaerobic treatment
a COD/S[O.sub.4.sup.2-] ratio higher than 10 is necessary [8]. Lower
ratios were thought to be detrimental to methanogenesis because they led
to the production of excessive sulphide concentrations (> 150-200 mg
[L.sup.-1]). In the experiment the sulphide concentrations in the
effluents of neither ASBR nor CSBR exceeded the inhibitory levels (150
mg [L.sup.-1], [9]) despite the fact that the COD/S[O.sub.4.sup.2-]
ratio of the influent was always lower than 8. The effluent sulphide
concentration was lower than 41.4 mg [S.sub.2][L.sup.-1].

During the steady state period of operation (days 39-89) the rate
of biogas production varied between 2.30 and 3.85 L [d.sup.-1]. This
indicated that the performance and functioning of the reactor were
rather variable. The variability observed was caused most probably by
competition between sulphidogens and methanogens and possible inhibitory
influence of sulphides (average value 18.85 mg [S.sub.2] [L.sup.-1]),
although they did not exceed the inhibitory level.

The composition of biogas was measured on the 68th day of the
experiment and was as follows: 60% C[H.sub.4], 35% C[O.sub.2], 2.7%
[H.sub.2]S. This composition indicated that mainly methanogenic
mineralization of organic matter was taking place in the ASBR. The
biogas production rate during the operation cycle was measured on the
50th day of the experiment. The data obtained showed that the rate of
biogas production was the greatest at the start of the cycle (during the
first 7 h after the period of raw water input), and then slowly
decreased with time, reaching very low and relatively stable levels at
the end of the reaction stage. Biogas production completely stopped in
the reactor on the 22nd-23rd hour of the cycle. The data showed that the
lengths of the stages of the treatment cycle had been chosen correctly.

Previous results have indicated that inoculation of UASB with
non-sulphate-adapted sludge could lead to complete inhibition of the
treatment process [2] because bacterial groups, especially methanogens,
could not adapt to the high levels of sulphide present in the influent.
However, in our research full inhibition of the process did not take
place. This could be explained by the presence of noncompetitive
substrates for methanogens (trimethylglycine) in yeast wastewater. Since
trimethylglycine remains undetected by a COD dichromate assay, its
concentration can be underestimated, which in turn may lead to a
significant overloading of WWTPs. It is known that sugarbeet molasses
used as a component of the growth medium for baker's yeast [5] in
the Salutaguse yeast plant contains up to 6% w/w trimethylglycine. In
anaerobic treatment plants, trimethylglycine is practically totally
degraded through a multistep degradation process with the formation of
nitrogen-containing intermediates--trimethylamine and other methylated
amines [10]. These intermediates are further degraded by methanogenic
bacteria, yielding C[O.sub.2], ammonium, and methane. The presence of
trimethylglycine could allow methanogens to maintain a significant
population in a sulphate containing environment, which stimulates the
growth of sulphate reducing bacteria (SRB), competitors of methanogens
for the same substrates in the anaerobic treatment processes.
Degradation of trimethylglycine (trimethylglycine is a nitrogenous
compound, whose complete anaerobic degradation can result in an increase
of the effluent ammonia concentration) and formation of amines can
explain also accumulation of [N.sub.tot] during the experiments carried
out by us (see Table 2).

As seen from Table 2, the effluent concentrations of Ntot increased
on average from 236 to 570 mgN [L.sup.-1]. It should be noted that
removal of all nitrogen compounds would require anaerobic,
microaerophilic, and aerobic conditions established simultaneously in
different locations of the anaerobic reactor, which is highly improbable
in the case of the small-scale laboratory vessels used in our
experiments.

The results of the present study (Table 2) demonstrate the ability
of the ASBR process to achieve a good phosphorus removal efficiency--up
to 61%. As calcium chloride is used in the technological process of
yeast production, wastewaters are characterized by a rather high content
of calcium ions. Under these conditions the high phosphorous removal
efficiency could be explained by precipitation as a result of the
formation of insoluble [Ca.sub.3][(P[O.sub.4]).sub.2].

Despite the high sulphate treatment efficiency achieved in the
ASBR, sulphide production during the process was significant, and this
led to the observed instability of the process. In the large-scale
experiments the instability of the processes could create significant
difficulties in applying the ASBR technology for the treatment of yeast
wastewaters. In addition to the inhibition of the process, sulphide
formation caused also major malodour problems and corrosion of equipment
during the experiment. In the further experimental work two
modifications of the ASBR technique were investigated to reduce the
problems noted. Accumulation of sulphides was an indication that
competition between methanogens and SRB was won by the latter.

The aim of the further investigation was to find experimental
conditions where methanogens would prevail, and the reduction of
sulphate would stop at the level of elemental sulphur. An ASBR with a
polymeric filler and coupled microaerophilic/anaerobic sequence batch
reactor (CSBR) were investigated.

ASBR with a polymeric filler

Experiments with a polymeric filler used as a support material for
microorganisms were performed in order to study the influence of an
artificial filler on the efficiency of the process. Previous studies
[11] had shown that the use of a support material favours the adherence
of methanogenic bacteria and accelerates the washout of SRB. A poor
attachment ability of SRB was demonstrated. It was concluded on the
basis of the experimental results that in the presence of a filler SRB
are washed out of the reactor providing acetotrophic methanogenic
bacteria with a sufficient growth advantage. These data suggest that an
artificial carrier could stimulate methanogenic activity in the
anaerobic digester and increase the efficiency and stability of the
treatment processes.

Two reactors were operated in our series of experiments during 68
days. One was loaded with a polymeric filler and the other was operated
like the first one but without the filler. The operational conditions
were the same as described in the previous experiments. The COD and
sulphate removal efficiencies were not significantly different between
the two reactors studied; however, in the reactor without the carrier a
slightly higher average treatment efficiency was observed, sulphate
removal efficiencies varied from 85% to 100%. The sulphide
concentrations in the effluents of either reactors did not exceed
inhibitory levels and were not higher than 123 and 110 mg [L.sup.-1],
respectively. These data are in agreement with the results of our
previous experiment. The efficiency of phosphorous removal in the
reactor with the carrier was significantly higher (up to 79%) than in
the control reactor (57%). It can be assumed that the carrier promoted
deposition of insoluble materials, for example precipitation of
[Ca.sub.3][(P[O.sub.4]).sub.2]. This conclusion was supported by the
observation that scaling of the carrier beads was observed in the
experiment. The fast clogging of the system with a carrier when treating
sulphate-rich wastewaters has been described also in several other
studies [12, 13]. In addition to facilitating scaling, a carrier could
hamper equal distribution of wastewater over the sections of the
reactor, which could result in a lower COD and sulphate treatment
efficiency. Therefore it can be concluded that the application of the
carrier for the given treatment system was not effective and cannot be
recommended.

Coupled microaerophilic/anaerobic system (CSBR)

In the CSBR the effluent from the anaerobic reactor was recycled
through an aeration system. The content of oxygen in the microaerophilic
reservoir was kept at the level of 0.1-0.15 mg [L.sup.-1] to prevent
sulphate formation in the oxidation of the sulphide formed in the
anaerobic stage of the process leaving sulphur in the form of elemental
sulphur ([S.sup.0]) [14]. It was assumed to be the best for simultaneous
solution of two problems: sulphate and sulphide removal. The formation
of elemental sulphur is an advantage because sulphur is a colloid, inert
solid and can be removed from the wastewater for example by gravity
sedimentation. The anaerobic reactor was seeded with sulphate adapted
anaerobic sludge, and the microaerophilic reactor was seeded with
activated sludge obtained from the full-scale aerobic reactor of the
Salutaguse yeast plant, Estonia.

The CSBR was operated during 68 days under the operational
conditions described in previous experiments. The maximum OLR achieved
was 7.74 kgCOD [m.sup.-3] [d.sup.-1]. The average pH value of the final
effluent was 8.2 and the alkalinity always remained up to 177 mEq
[L.sup.-1] at the average pH of the influent 4.2. No attempts were made
to adjust the pH of the influent. High pH values could be explained by
the formation of hydroxide ion during the following biological overall
reaction, taking place in a microaerophilic sulphide removal system
[15]:

2H[S.sup.-] + [O.sup.2] [right arrow] 2[S.sup.0] + 2O[H.sup.-]. (2)

The results obtained allow us to conclude that a rather good COD
removal efficiency (50-70%) was achieved during the experiment. Since
the sludge had been well adapted to wastewater a very quick start-up was
observed. Only a few days after seeding, the COD removal significantly
increased and reached 70%.

The optimal COD loading found for the ASBR was 6-8 kgCOD [m.sup.-3]
[d.sup.-1]. The highest COD removal efficiencies, exceeding 65% in the
CSBR, were observed at the same (from 6 to 8 kgCOD [m.sup.-3]
[d.sup.-1]) OLR values (Fig. 6).

Taking into consideration that the optimal ORL value reported in
the literature [1] for different methanogenic reactors varies
remarkably--from 4 to 12 kgCOD [m.sup.-3] [d.sup.-1]--the results
obtained in our experiments were quite good for the treatment of high
strength sulphate-rich wastewaters. The sulphate removal efficiency
achieved in our experiments was excellent--more than 98%. Due to the low
dissolved oxygen concentration (0.1-0.15 mg [L.sup.-1]) there were
almost no sulphides and sulphates in the effluent (Fig. 7). Only
approximately 0.5 mg [L.sup.-1] of [H.sub.2]S and 0-30 mg [L.sup.-1] of
S[O.sub.4.sup.2-] were present in the effluent while up to 3.6 g
[L.sup.-1] of sulphate had been reduced.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Our data suggest that keeping a low level of the dissolved oxygen
concentration in the microaerophilic part of the treatment system helps
to poise the treatment process towards the formation of elemental
sulphur and that the coupled microaerophilic/anaerobic treatment
processes of sulphate-containing wastewaters were effective in
alleviating sulphide inhibition of both methanogenesis and sulphate
reduction. Last but not least, the exceptional stability of the CSBR
process should be noted. The operational conditions worked out in the
laboratory-scale experiments were successfully applied at full scale in
the Salutaguse yeast plant, where the process has been applied by now
for more than a year.

Final sludge tests

Microscopic examination of the sludge and of the biomass
concentration were performed at the beginning and at the end of each
experiment. In none of our experiments granulation was detected.
However, significant changes in the structure of the sludge were
recorded.

Serious scaling of biomass by inorganic precipitation was observed
already during 3.5 months of operation. Measurements of biomass
concentration showed that the density of the sludge had also
significantly changed since the start of the experiments. The sludge
concentration varied between 43.2 g TS [L.sup.-1] at the beginning of
the experiment and 62 g TS [L.sup.-1] in the ASBR and 65.2 g TS
[L.sup.-1] in the CSBR at the end of the study (Fig. 8). The difference
between the values of total solids and volatile suspended solids
indicated the presence of inorganic salts in suspension, possibly
calcium carbonate and phosphates. Due to the formation of elemental
sulphur in the CSBR a faster accumulation rate of inorganic compounds
was observed than in the ASBR. With all advantages of this type of
reactor the fast accumulation of inorganic compounds is an essential
disadvantage. Precipitation of inorganic salts, as for example calcium
carbonate, can indirectly upset the reactor performance by scaling [6,
16], which interferes with a good mass transport of substrate and
reaction products. Scaling of biomass by Ca precipitates (CaC[O.sub.3]
and/or [Ca.sub.3][(P[O.sub.4]).sub.2]) may already occur at [Ca.sup.2+]
concentrations of 400 mg [L.sup.-1] [17]. Also clogging problems can
arise from precipitates in the piping system. Unfortunately, the
concentration of calcium in the influent and effluent was not measured
in the present study and the problem of the formation and removal of
inorganic precipitate requires more detailed study in the future.

[FIGURE 8 OMITTED]

CONCLUSIONS

The results of the study carried out demonstrated that the
anaerobic sequencing batch reactor (ASBR) is a suitable and effective
tool for anaerobic treatment of sulphate-rich wastewaters from
baker's yeast production plants. Optimal parameters of the process
were determined. However, sulphide formation caused significant malodour
problems and corrosion of equipment during the experiment. Experiments
with two additional schemes developed for solving the sulphide formation
problem showed that use of plastic carriers in the reactor led to a
decrease of the treatment efficiency due to the accumulation of
insoluble sediment (presumably CaC[O.sub.3] and
[Ca.sub.3][(P[O.sub.4]).sub.2]) on the surface of the carriers. So this
technology cannot be recommended for large-scale application.

Combination of anaerobic sulphate reduction with biological
oxidation of sulphide in a coupled microaerophilic/anaerobic SBR (CSBR)
showed the best results and might be preferable for the treatment of
sulphate-rich yeast wastewaters. As the scaling of biomass and fast
accumulation of inorganic compounds were observed also in this case,
successful application of the CSBR technology requires finding a
solution for the removal of the inorganic precipitate from the reactor.
The data obtained by us will be useful in designing full-scale ASBR and
CSBR processes.

ACKNOWLEDGEMENT

The authors are grateful to AS Salutaguse Parmitehas, Estonia, for
supporting this study.